zlua: use REPLACE_LUA on z.cmd, and add an explanatory comment

zlua/MESSAGE

@@ -4,7 +4,7 @@ $NetBSD$
 To begin using z.lua, put something like the following in
 either ~/.profile or ~/.bashrc:
 
-if [ -e /usr/pkg/bin/z.lua ]; then
+if [ -e ${PREFIX}/bin/z.lua ]; then
     eval "$(lua ${PREFIX}/bin/z.lua --init posix)"
 fi
 

zlua/Makefile

@@ -13,6 +13,7 @@ COMMENT=	Tool that lets you quickly jump around your filesystem
 LICENSE=	mit
 
 NO_BUILD=	yes
+REPLACE_LUA+=	z.lua
 
 # Upstream has no releases, so we don't use ${DISTNAME} here.
 WRKSRC=	${WRKDIR}/${GITHUB_PROJECT}-${GITHUB_TAG}
@@ -29,6 +30,7 @@ do-install:
 	${INSTALL_DATA} ${WRKSRC}/README.cn.md ${DESTDIR}${PREFIX}/share/doc/${PKGBASE}
 
 	# z.cmd is a Batch/.bat script for Windows users. Not terribly useful for Unix-ey users, so into share/ it goes.
+	# Note that we don't use ${INSTALL_SCRIPT} here since being +x'd wouldn't do anything on Windows.
 	${INSTALL_DATA} ${WRKSRC}/z.cmd ${DESTDIR}${PREFIX}/share/${PKGBASE}
 
 .include "../../lang/lua/application.mk"